The present invention relates to a magnetic sheet, and more particularly, to a magnetic sheet which is excellent in flexibility, mechanical properties, storage stability and product stability, and wherein the content of magnetic particles can be considerably enhanced notwithstanding the amounts of plasticizer and lubricant used therein are considerably minimized.
Magnetic sheets have been produced by molding a composition comprising a binder and magnetic particles as a filler into a sheet having a thickness of 0.1 to 10 mm, and then magnetizing the obtained sheet. These magnetic sheets have been widely used in various application fields such as marks for sign or display, beginner's marks for automobiles, various fitting or attaching sheets as stationaries or office supplies, or sealing materials for doors, e.g., door packings for refrigerators.
In the case where the magnetic sheet is fitted or attached to various objects, it is necessary that the magnetic sheet is prevented from being slipped off or fallen off from the predetermined position. Further, when the magnetic sheet is used as a sealing material for doors, it is necessary that the door can be brought into close contact with a body of equipment. In order to satisfy the above-described requirements, it has been demanded to enhance a filling property of magnetic particles in the magnetic sheet, i.e., to increase a content of magnetic particles in the magnetic sheet, thereby obtaining a large magnetic flux density on the surface of the magnetic sheet.
In general, the articles or objects to which the magnetic sheet is to be fitted or attached, have not only simple flat surfaces but also irregular, partially sloped or complicated surfaces. In order to allow the magnetic sheet to be fitted or attached onto such various surfaces in a close contact manner, and to be readily detached therefrom, the magnetic sheet has been required to exhibit a good flexibility.
Also, it has been required that the magnetic sheet is free from partial breakage or tear even after repeatedly attached and detached for a long period of time, namely the magnetic sheet has been required to show an excellent mechanical strength.
From the description in Japanese Patent Application Laid-Open (KOKAI) No. 11-49904(1999): "as sound-insulating materials, there have been proposed sheets prepared by adding a filler having a high specific gravity to a rubber component. These sound-insulating materials are stored in stacks at factories or building sites after the production thereof until these materials are applied to apartment houses or individual detached houses. Here, there arises such a problem that the stacked sheets suffer from blocking therebetween during the storage, especially under high-temperature and high-humidity conditions such as in the summer season, because the rubber component is contained therein."; and in Japanese Patent Application Laid-Open (KOKAI) No. 11-90991(1999): "as damping sheets used for a folding plate-type roof, there have been proposed composite damping materials comprising an elastomer and inorganic particles. However, in the case where the composite damping material is wound up into a roll upon the production thereof, it has been sometimes difficult to draw the sheet out of the roll because the composite damping material itself has a stickiness. In particular, under high-temperature conditions such as during the summer season or the like, such a tendency becomes remarkable. As a result, there arises such a problem that portions of the rolled composite damping material are adhered together, so that it is not possible to draw the sheet out of the roll.", in the case where magnetic sheets are preserved or stored in stacks, there tends to be caused such a phenomenon that the surfaces of these magnetic sheets are stuck together due to a rubber component contained in the resin, namely so-called blocking between the sheet is likely to occur. For this reason, it has been demanded to provide a magnetic sheet which is free from blocking during preservation or storage, i.e., is excellent in so-called storage stability.
Also, when the magnetic sheet is kept fitted or attached onto any objects or articles for a long period of time, the plasticizer contained in the magnetic sheet is bled out, so that the stickiness of the magnetic sheet is increased. In this condition, when the magnetic sheet is detached from the surfaces of the objects or articles, there tends to arise such a phenomenon that the surfaces of the objects or articles are contaminated with a sticky material peeled off from the rear surface of the magnetic sheet, i.e., so-called fogging phenomenon is likely to occur. Therefore, it has also been demanded to provide a magnetic sheet which is free from any contamination even when being kept fitted or attached for a long period of time, namely is excellent in so-called product stability.
As conventional magnetic sheets exhibiting an enhanced filling property of magnetic particles and a good flexibility, there have been widely used magnetic sheets in which a chlorine-containing resin such as, typically, polyethylene chloride, vinyl chloride, chlorosulfonated polyethylene or the like is used as a binder, and a plasticizer and a lubricant are mixed or blended therein in amounts of 1 to 120 parts by weight and 1 to 30 parts by weight, respectively, based on 100 parts by weight of the total amount of the binder.
In the above Japanese Patent Application Laid-Open (KOKAI) No. 11-90991(1999), there has been described the magnetic sheet using a styrene-isoprene block copolymer and an ethylene-vinyl acetate copolymer as a binder.
Also, European Patent Publication No. 0 690 092 A1 discloses a damping material comprising:
(a) a binder resin composed of 85 to 99% by weight of at least one base resin selected from polypropylenes, polystyrenes, acrylonitrile-butadiene-styrene copolymers, polycarbonates, polyphenylene ethers and modified polyphenylene ethers, and 1 to 15% by weight of styrene-isoprene-styrene block copolymer; and PA1 (b) 1 to 20 parts by weight, per 100 parts by weight of the binder resin, of iron compound particles. PA1 400 to 1,900 parts by weight of at least one kind of magnetic particles selected from the group consisting of magnetoplumbite-structure ferrite particles and rare-earth magnet particles as a filler; and PA1 100 parts by weight of a binder comprising: PA1 400 to 1,900 parts by weight of at least one kind of magnetic particles selected from the group consisting of magnetoplumbite-structure ferrite particles and rare-earth magnet particles as a filler; and PA1 100 parts by weight of a binder comprising: PA1 1) Resin blends comprising polyolefin such as polypropylene or polyethylene as a hard segment, and an ethylene-propylene rubber as a soft segment; PA1 2) Resins prepared by polymerizing polypropylene or polyethylene as a hard segment with an ethylene-propylene rubber as a soft segment; or PA1 3) Resins prepared by vulcanizing a soft segment in a blend comprising polypropylene or polyethylene as a hard segment and an ethylene-propylene rubber as the soft segment.
It has been demanded to provide a magnetic sheet which is not only excellent in flexibility, mechanical strength, storage stability and product stability, but also can be considerably enhanced in content of magnetic particles. However, such magnetic sheets cannot be obtained yet.
Namely, in the above-described conventional magnetic sheet using the chlorine-containing resin as a binder, it is essentially required to not only increase the content of magnetic particles but also add a so-called plasticizer for imparting a plasticity and processability to the magnetic sheet, so that the plasticizer added tends to be bled out with the passage of time. For this reason, the conventional magnetic sheet is deteriorated in product stability as well as storage stability.
In addition, in the case where a lubricant is added in order to enhance the flowability of the chlorine-containing resin, the filling property, mechanical strength and product stability of the magnetic sheet can be improved. However, there arises such a problem that the magnetic sheet is extremely deteriorated in flexibility.
As is known in the art, chlorine-containing resins tend to generate harmful gases such as chlorine gas during processing thereof and, therefore, adversely affect the working environments. Also, when the chlorine-containing resins are incinerated upon disposal, harmful substances such as dioxin are produced, so that there is caused a problem that the environmental conditions are adversely affected.
Further, in the case of the magnetic sheet described in the above Japanese Patent Application Laid-Open (KOKAI) No. 11-90991(1999), when the content of magnetic particles is increased, the storage stability and product stability of the obtained magnetic sheet becomes deteriorated, as described in Comparative Examples hereinlater.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that by using as a binder, a resin composition comprising 50 to 95% by weight of a thermoplastic elastomer of a styrene-isoprene-styrene block copolymer, a styrene-ethylene-butylene-styrene block copolymer and/or an ethylene-propylene copolymer rubber, and 5 to 50% by weight of a soft polyolefin of a propylene homopolymer and/or a propylene-ethylene copolymer, the obtained magnetic sheet is not only excellent in flexibility, mechanical strength, storage stability and product stability but also exhibits a high filling property of magnetic particles, and is free from the generation of harmful substances such as dioxin even when subjected to incineration upon disposal, since no chlorine-containing resin is used as a binder therein. The present invention has been attained on the basis of this finding.